1Introduction

Since the age of alchemy and the search for the

philosopher’s stone, man has looked for ways of

controlling the transformation of matter. Today,

chemists seek to control the outcome of chemical

reactions, to suppress unwanted side products and to

synthesize new molecules. In this book we will see

how this long-standing dream has been partially

achieved through the application of lasers in chemis-

try and how sometimes we can even teach lasers to be

as skilful as chemists!

1.1 Basic concepts in laserchemistry

The laser has revolutionized many branches of

science and technology, and this revolution is seen

very clearly in chemistry, where the laser has now

become one of the essential tools of chemistry.

Chemistry is a scientific discipline that studies

matter and its transformation, and it is precisely in

these two areas that lasers and laser technology play

such a crucial role. As illustrated in Figure 1.1, the

laser is a powerful tool which can be used to charac-

terize matter by measuring both its properties and

composition. Furthermore, the use of laser radiation

can be a powerful method to induce or probe the

transformation of matter in real time, on the femtose-

cond (10�15 s) time-scale.

The links between the laser and chemistry

The links between the laser and chemistry are mani-

fold, as shown schematically in Figure 1.2.

First, we have the so-called chemical lasers. This

link goes directly from chemistry to lasers, i.e. a

chemical reaction provides the energy to pump a

laser. An example of this type of laser is the HF

chemical laser, in which fluorine atoms, produced

in a discharge, react with H2 to produce a population

inversion in the ro-vibronic states of the product HF:

Fþ H2�!HFz þ H

The excited HFz then produces intense, line-tuneable

laser output in the infrared (IR).

Another example is the excimer laser, where ion-

molecule and excited state reactions produce a popu-

lation inversion; e.g. in the KrF laser, an electric

discharge through a mixture of Kr and F2, diluted in

He, produces Krþ ions, Rydberg-excited Kr*, and F

atoms, which subsequently react, yielding excited-

state KrF*. Since the ground-state potential is repul-

sive (i.e. the ground state is unbound) the molecule

Laser Chemistry: Spectroscopy, Dynamics and Applications Helmut H. Telle, Angel Gonzalez Urena & Robert J. Donovan# 2007 John Wiley & Sons, Ltd ISBN: 978-0-471-48570-4 (HB) ISBN: 978-0-471-48571-1 (PB)

COPYRIG

HTED M

ATERIAL

dissociates immediately after emitting a photon and

population inversion is ensured. Excimer lasers are

nowadays widely used in the car industry for welding,

in research laboratories for fundamental and applied

science, and in ophthalmology for eye surgery, to

name the most common applications.

These are just two examples where the energy from

a chemical reaction is transformed into coherent

radiation that is subsequently employed in various

applications.

A second link involves the use of lasers as ‘analy-

tical’ tools, for sample analysis and characterization.

In this wide field of analytical applications, lasers have

been used to probe a variety of systems of chemical

interest. Both stable species and nascent radicals pro-

duced by fast chemical reactions can be monitored

with high sensitivity. As we shall see in later chapters,

the special properties of laser radiation have opened

up many new possibilities in analytical chemistry.

The third link between lasers and chemistry is the

initiation and control of chemical change in a given

system. The initiation of chemical processes by laser

radiation has become a powerful area, not only in

modern photochemistry, but also for technological

applications. An example within this category is the

multiphoton dissociation of SF6 (sulphur hexafluor-

ide) by IR laser radiation; this provides a means by

which the two isotopomers 32SF6 and 34SF6 can be

separated.

A further example is the control of chemical reac-

tions using the methods of coherent control; this

approach lies at the cutting edge of current research

in laser chemistry, and we will discuss this topic in

some detail in Chapter 19.

The laser: a ‘magical’ tool in analyticalchemistry

Undoubtedly, lasers have become a sort of a ‘magical’

tool in analytical chemistry. So, why is this? We

Figure 1.1 Principle processes in laser chemistry

LASER CHEMISTRYlaser instrumentation

probing matterof chemical interest

process controlinduced chemical change

chemical laserslaser materials

Figure 1.2 The interconnections between chemistry andlaser technology

2 CH1 INTRODUCTION

could answer in a variety of ways, but perhaps the

easiest way is to address the specific properties

that characterize laser radiation. Briefly, the proper-

ties that distinguish lasers from ordinary light

sources are:

� they are brighter;

� they are tuneable and highly monochromatic;

� they are highly directional;

� they allow polarization control;

� they are temporally and spatially coherent;

� they can probe molecules on the femtosecond

(10�15 s) time-scale.

The brightness of a laser not only implies a better

signal-to-noise ratio, but more importantly the cap-

ability of probing and recording trace concentrations

of transient species, reaction intermediates, photodis-

sociation fragments, etc. In fundamental research,

all methods for probing reactions require single-

collision conditions; under these conditions, the

high laser power makes up for the low particle

density. In addition, powerful lasers open up new

dimensions in non-linear phenomena, i.e. two-photon

or multiphoton processes leading to dissociation and/

or ionization.

Laser radiation is monochromatic and in many

cases it also is tuneable; these two characteristics

together provide the basis for high-resolution laser

spectroscopy. The interaction between laser radiation

and molecules can be very selective (individual quan-

tum states can be selected), permitting chemists to

investigate whether energy in a particular type of

molecular motion or excitation can influence its

reactivity. Photochemical processes can be carried

out with sufficient control that one can separate

isotopes, or even write fine lines (of molecular dimen-

sions) on surfaces.

The output of most lasers can be, or is, polarized.

The polarization character of the laser field interact-

ing with the chemical reaction partners is indispensa-

ble when investigating stereodynamic effects. For

example, by changing the plane of polarization

of the laser radiation used to excite a reagent, the

symmetry of the collision geometry can be altered

and, consequently, the outcome of a chemical reaction

may change (see the brief examples below in the

‘Stereodynamical aspects’ section).

Temporal coherence allows laser pulses to be tai-

lored, providing the chemist with the opportunity to

observe rapid changes down to the femtosecond time-

scale. Using the technique of femtosecond excitation

and probing, we now have the capability to study

ultrafast reactions in real time.

The coherent character of laser radiation is

reflected in the photon distribution function, whose

phase relation distribution is peaked and very narrow,

in contrast to the distribution function from a chaotic

light source. Therefore, within a small interaction

volume, the spatial and temporal coherence in the

laser field results in significant transition probabilities

for multiphoton absorption processes, whose occur-

rence would be nearly insignificant using an incoher-

ent radiation source, even one exhibiting the same

overall irradiance as a coherent laser source. This

difference in transition probability is crucial for the

successful implementation of any method requiring

multiphoton absorption, both in general molecular

spectroscopy (in monitoring chemical processes)

and in techniques exploited in laser analytical chem-

istry. One such technique is resonance-enhanced mul-

tiphoton ionization (REMPI).

One of the major problems in analytical chemistry

is the detection and identification of non-volatile

compounds at low concentration levels. Mass spectro-

metry is widely used in the analysis of such com-

pounds, providing an exact mass, and hence species

identification. However, successful and unequivocal

identification, and quantitative detection, relies on

volatilization of the compound into the gas phase

prior to injection into the analyser. This constitutes a

major problem for thermally labile samples, as they

rapidly decompose upon heating. In order to circum-

vent this difficulty, a wide range of techniques have

been developed and applied to the analysis of non-

volatile species, including fast atom bombardment

(FAB), field desorption (FD), laser desorption (LD),

plasma desorption mass spectrometry (PDMS) and

secondary-ion mass spectrometry (SIMS). Separating

the steps of desorption and ionization can provide an

important advantage, as it allows both processes to be

1.1 BASIC CONCEPTS IN LASER CHEMISTRY 3

optimized independently. Indeed, laser desorption

methods have recently been developed in which

the volatilization and ionization steps are separated,

providing very high detection sensitivity.

REMPI, coupled with time-of-flight mass spectro-

metry (TOFMS), is considered to be one of the most

powerful methods for trace component analysis in

complex mixtures and matrices. The high selectivity

of REMPI–TOFMS stems from the combination

of the mass-selective detection with the process of

resonant ionization; thus, absorption of two or more

laser photons through a resonant, intermediate state

provides a high level of selectivity, (i.e. laser wave-

length-selective ionization). The main advantages of

REMPI–TOFMS are its great sensitivity and resolu-

tion, high ionization efficiency, the control of mole-

cular fragmentation (by adjusting the laser intensity

appropriately), and the possibility of simultaneous

analysis of different components present in a given,

complex matrix (e.g. non-volatile compounds in

biological samples).

Lasers and chemical reactions

Figure 1.3 shows schematically the different regimes

where laser techniques can be applied in the study of

chemical reactions. Note that we have made a clear

distinction between gas- and condensed-phase pro-

cesses, but only for pedagogical purposes (the tech-

nique and fundamental interactions underlying the

overall phenomena are frequently the same). For

the same reasons, we have separated unimolecular

from bimolecular processes in later chapters. We

also separately address condensed-phase processes,

like surface chemistry, solution reactions, photo-

biochemistry, hydrodynamics and etching, in the

chapters that follow.

The use of lasers to prepare reactants and/orprobe products of chemical reactions

A chemical reaction can be viewed as dynamical

motion along the reaction coordinate from reactants

to products. Thus, lasers can be used to prepare

reagents and to probe products in particular quantum

states.

A good example of the first category is the enhance-

ment of a chemical reaction following vibrational

excitation of a reagent. The first experiment showing

that vibrational excitation can enhance the cross-

section of a chemical reaction was reported for the

crossed-beam reaction

Kþ HCl�!KClþ H

Figure 1.3 How lasers can be employed to pump, influence and probe chemical reactions

4 CH1 INTRODUCTION

An HCl chemical laser was employed to resonantly

excite the v00 ¼ 1 level of the HCl reactant. It was

estimated that, following vibrational excitation of

HCl, the KCl yield was enhanced by about two orders

of magnitude.

On the other hand, lasers can also be used to probe

reaction products. A representative example is that of

reactions of the type

MðM ¼ Mg;CaÞ þ X2ðX ¼ F;ClÞ�!MXz þ X

in which the nascent MXz can be probed by laser-

induced fluorescence (LIF). Indeed, rotational and

vibrational product state distributions have been

determined for this type of reaction from the analysis

of such LIF spectra. The reaction is known to occur via

electron transfer from the metal atom to the di-

halogen. The negative ion X�2 so formed rapidly dis-

sociates under the Coulombic attraction of the Mþ

ion. LIF analysis of the nascent CaCl versus MgCl

product indicates that whereas CaCl is formed vibra-

tionally excited, MgCl is only rotationally excited.

This difference can be explained by the difference in

the range at which the electron jump takes place.

Whereas in the Ca reactions the jump occurs at long

range such that energy is channelled into the Ca—X

coordinate (i.e. as CaCl vibrational energy), in the Mg

reaction the electron jump distance is shorter, such

that there is no possibility for vibrational excitation of

the product and most of the energy appears in rota-

tional excitation of the MgCl. This is, therefore, a

clear example in which laser probing of the nascent

reactionproducthelps tounravel the reaction mechan-

ism and dynamics at a detailed molecular level.

Probing product state distributions by multiphoton

ionization is one of the most sensitive methods for

the analysis of both bimolecular and photofragmenta-

tion dynamics. For example, by using REMPI one

can measure the rotational state distribution in the

N2 fragment produced in the photofragmentation of

N2O; it was found that the maximum in the rotational

state population is near J 70. This reveals that

although the ground electronic state is linear, the

excited state is bent and thus the recoil from the O

atom results in rotational excitation of the N2

molecule.

It is often possible to use lasers to pump and probe a

chemical reaction simultaneously. In other words, it is

possible to use one laser to prepare a reactant in a

specific quantum state and a second laser to probe the

product. A good example is the reaction

Ca�ð4s4p1P1Þ þ H2�!CaHðX2�þÞ þ H

which is exothermic by 1267 cm�1. On the other

hand, the ground-state reaction

Cað4s2Þ þ H2�!CaHðX2�þÞ þ H

is endothermic by 22 390 cm�1. Therefore, two

lasers were needed to investigate the reaction dyna-

mics in this case: a pump laser operating at

lexc¼ 422.7 nm was used to prepare Ca atoms in

the 1P1 state and a probe laser was used to excite LIF

from the CaH via the B2�þ�X2�þ transition (in

the wavelength range of 620–640 nm). The (0, 1)

and (1, 2) transitions were monitored and analysis

of the rotational line intensities gave the product

rotational distribution (given the Honl–London

factors, the rotational line intensities for the v ¼ 0

and v ¼ 1 levels could be determined). Furthermore,

by summing up the rotational lines for each level,

and by making use of the Franck–Condon factors,

the CaH vibrational distribution could be deduced.

Laser-assisted chemical reactions

Laser assisted chemical reactions are defined as reac-

tions in which the product yield is enhanced by excit-

ing the transition state (i.e. the reactants are not

excited). A classical example for a photon-mediated

atom–diatom reaction is that of

Kþ NaCl�!jKCl Naj6¼ þ h��!KClþ Na�

Excited Na* is observed when the laser photon is

selected to excite the jKCl Naj6¼ transition state:

the excitation photon does not have the correct

(resonant) energy to excite either K or NaCl (see the

conceptual diagram in Figure 1.4).

Laser-stimulated versus laser-inducedchemical reactions

Chemical reactions can be stimulated or induced by

lasers. The former case refers to the situation where

1.1 BASIC CONCEPTS IN LASER CHEMISTRY 5

the laser enhances the reaction rate. Thus, when the

laser is turned off, the reaction rate diminishes but the

process continues. The example given above, namely

the reaction K þ HCl! KClþH, which can be

stimulated with a chemical laser by pumping the

v00 ¼ 1 level of HCl, thus falls into this category.

The situation is different, however, for a laser-induced

chemical reaction, e.g. the multiphoton dissociation

of SF6, leading to the products SF5þ F. When the

laser is turned off, photodissociation ceases.

Gas-phase photodissociation can be induced

by single- or multi-photon excitation processes.

Single-photon dissociation, combined with imaging

techniques, has revealed detailed insight to the bond-

breaking process. A representative illustration of

this type of study is the far-ultraviolet (UV) photolysis

of NO2 leading to the products O(1D)þNO. From

the analysis of the imaging data for O(1D), both

the translational and the vibrational distributions

in the product fragments have been deduced; these

data provide a detailed insight into the dynamics

of the dissociation process and clearly show that

there is a change in geometry, from bent to linear, on

excitation.

Molecular photodissociation can also be achieved

by IR multiphoton excitation. A classical example

of such processes is that of SF6þ nh� ! SF5þ F,

which can also be applied to produce isotope

enrichment in a 32SF6/34SF6 mixture (the mecha-

nism for this process is discussed in some detail in

Chapter 18).

Stereodynamical aspects

The electronic excitation of a reagent can have several

effects on a chemical reaction. For example, a higher

electronic energy content in a reagent can make a

reaction exothermic that would otherwise have been

endothermic; as a consequence, enhancement of the

reaction yield may ensue. However, laser excitation of

a reactant species (atom or molecule) not only

increases its internal energy, it also generally modifies

its electronic state symmetry. It is well known that

symmetry plays an important role in photon-induced

transitions (cf. selection rules in electronic, vibra-

tional and rotational spectroscopy), but it can also

play an important role in chemical reactivity. Electro-

nic excitation invariably changes the shape and

symmetry of the potential energy surface (PES) and

it may induce a different reaction mechanism com-

pared with that of the ground state. An example is the

change from abstraction to predominantly insertion

reactions, seen for oxygen atoms, as one goes from the

ground state, O(3P), to the first excited state, O(1D).

Here also, we see that energy alone is not sufficient to

promote a reaction, as the second excited state of the

oxygen atom, O(1S), is far less reactive than the lower

energy O(1D) state.

Since the early days of reaction dynamics, the

vectorial character of the elementary chemical reac-

tion has been well recognized. Not only are scalar

quantities (such as collision energy or total reaction

cross-section) important in shaping a reactive colli-

sion, but vectorial properties (such as the reagent’s

orientation, and orbital or molecular alignment) can

also significantly influence the outcome of an elemen-

tary chemical reaction.

For example, photodissociation is an anisotropic

process. The polarization of the electric field of the

photolysis laser defines a spatial axis, to which

the vectors describing both the parent molecule and

the products can be correlated (see Figure 1.5).

In a full-collisionexperiment, e.g. in crossed-beam,

beam–gas or gas cell arrangements, the reference axis

is the relativevelocity vector. Conceptually, thevector

correlation is identical to that of photodissociation,

only now the relative-velocity vector rather than the

electric-field vector defines the symmetry. Thus, the

reagents’ electronic orbital alignment can influence

the product yield of a chemical reaction. Imagine, for

hνpump‡

hνproducts

AB + C

A + BC

A⋅⋅⋅B⋅⋅⋅C ‡

A⋅⋅⋅B⋅⋅⋅C ‡

A⋅⋅⋅B⋅⋅⋅C ‡

A⋅⋅⋅B⋅⋅⋅C ‡

*

*

AB + C

endothermic

EN

ER

GY

Figure 1.4 Laser-assisted (endothermic) chemical re-action. Note that h�pump

z excites neither reagents norproducts

6 CH1 INTRODUCTION

example, the system Aþ B2, which yields the reac-

tion products ABþ B. In the case that the A atom is

elevated to an excited state, say by a transition1S0! 1P1, the alignment of the 1P1 orbital with

respect to the relative-velocity vector can influence

the outcome of the reaction A(1P1)þ B2! ABþ B.

For the case that the p-orbital is parallel to the relative

velocity vector, the PES is of so-called � symmetry

and the yield of AB in its excited � state is enhanced.

Conversely, if the p-orbital is perpendicular to the

relative velocity vector, then the yield of AB in its

electronically excited � state is enhanced, as shown

pictorially in Figure 1.6.

The direct correlation observed between the paral-

lel and perpendicular alignments in the centre of mass

and the �- and �-product channels, in the laboratory

frame, is an example of the stereodynamical aspect of

chemical reactions that can be precisely investigated

by linking them to suitable laser photon fields.

The universality of the laser chemistry

A complete knowledge of chemical reactivity

requires a full understanding of single collision

events. One of the most powerful tools with which

to investigate such events is the molecular beam

method. Under molecular beam conditions one can

study bimolecular reactions in great detail, using both

laser excitation and probing techniques.

The intermediates in many bimolecular reactions

exhibit lifetimes of less than a picosecond. Thus, it

was only after the development of ultrafast laser

pulses (of the order of 100 fs or so) that it has become

possible to study the spectroscopy and dynamics

of transitions states directly, giving rise to the so-

called field of femto-chemistry. This discipline has

revolutionized the study of chemical reactions in

real time, and one of its most prominent exponents,

Ahmed H. Zewail, was awarded the Nobel Prize for

Chemistry in 1999 for his pioneering contributions to

this field.

Chemical reactivity depends significantly on the

state (phase) and degree (size) of aggregation of a

particular species. Laser techniques have been devel-

oped to study chemical processes in the gas phase, in

clusters, in solutions and on surfaces. Thus, clusters,

i.e. finite aggregates containing from two up to 104

particles, show unique properties that allow us to

investigate the gradual transition from molecular to

Figure 1.5 Stereodynamical effects of polarized laserinteractions, here exemplified for laser-induced photo-fragmentation; spatial orientation of the initial rota-tional axis of the molecular product is induced. Toppanel: the laser polarization is out of the dissociationplane; bottom panel: the laser polarization is in the dis-sociation plane. The dashed lines indicate the centre-of-mass coordinate of the receding products

Figure 1.6 Stereodynamical effect as a result of laserorbital alignment of the reagent atom; spatial orientationof the atomic dipole moment is induced. Top panel: thelaser polarization is out of the collision plane; bottompanel: the laser polarization is in the collision plane. Thedashed lines indicate the centre-of-mass coordinate ofthe receding products

1.1 BASIC CONCEPTS IN LASER CHEMISTRY 7

condensed-matter systems (see the schematics of the

transition from isolated particles through aggregates

to solid surfaces in Figure 1.7).

The binding forces in aggregates and clusters are

often weak interactions of the van der Waals type.

These van der Waals forces are responsible for impor-

tant phenomena such as deviations from ideal gas

behaviour, and the condensation of atoms and mole-

cules into liquid and crystalline states. Such weakly

bound van der Waals molecules have become model

systems in chemistry. Both the structure and the

photodissociation of van der Waals molecules are

discussed later in some detail (see the examples in

Part 6).

The study of laser-induced chemical reactions

in clusters is normally carried out in a molecular

beam environment. One of the great advantages of

using the molecular beam technique is its capability

to generate supercooled van der Waals clusters of

virtually any molecule or atom in the periodic

table. This method of ‘freezing out’ the high number

of excited rotational and vibrational states of mole-

cular species in the beam is a powerful tool, not only

to implement high-resolution spectroscopic studies,

but also to form all kinds of aggregates and clusters.

One of the most widely used methods for cluster

formation is the technique of laser vaporization.

This powerful method was developed by Smalley in

the 1980s and led to the discovery of C60 and the other

fullerenes, which was recognized by the award of the

1996 Nobel Prize in Chemistry to Kroto, Curl and

Smalley.

Reactions in solution are very important in

chemistry; the solvent plays a crucial role in these

processes. For example, trapping reactive species in

a ‘solvent cage’ (see the centre part of Figure 1.7 for

a schematic of the principle), on the time-scale

for reaction, can enhance bond formation. The solvent

may also act as a ‘chaperone’, stabilizing energ-

etic species. Studies in solvent environments have

only become possible recently, once again aided

by the advent of ultrafast lasers, which allowed

the investigation of the solvation dynamics in real

time.

Processes such as photodissociation of adsorbed

molecules or phonon- versus electron-driven sur-

face reactions are topics that are currently attract-

ing great attention. The photodissociation of an

adsorbed molecule may occur directly or indir-

ectly. Direct absorption of a photon of sufficient

energy can result in a Franck–Condon transition

from the ground to an electronically excited repul-

sive or predissociative state. Indirect photodisso-

ciation of adsorbates, involving absorption of

photons by the substrate, can take place via two

processes. The first one is analogous to the process

of sensitized photolysis in gases (i.e. energy is

transferred from the initially excited species to

another chemical species). The second one, also

substrate mediated, implies the photo-transfer of

an electron from the substrate to an anti-bonding

orbital of the adsorbate, i.e. charge transfer photo-

dissociation. Laser techniques are now revealing

some of the fundamental principles involved in

these two excitation mechanisms.

Figure 1.7 The generalization of laser chemistry. Toppanel: laser-mediated gas-phase reaction; middle panel:laser-mediated reaction of a molecule trapped in a (cluster)solvent cage; bottom panel: laser-mediated reaction ofan adsorbed molecule with a surface atom/molecule (laserinteracts with adsorbed molecule or the surface)

8 CH1 INTRODUCTION

State-of-the-art laser chemistry

Probably the most revolutionary development in our

knowledge of the nature of the chemical bond and

the dynamics of the chemical reactions has been

gained by using ultrafast lasers, mostly in the fem-

tosecond time-scale. This area of research is now

commonly known as femto-chemistry, and ample

coverage is given to it in this book. As we will

show in detail later, laser excitation by femtosecond

pulses leads to a coherent superposition of excited

states. By observing the time evolution of the wave

packet that is created, one can record snapshots of

molecular photofragmentation and chemical reac-

tions, i.e. the bond-breaking and bond-forming pro-

cesses can be studied in real time.

Traditionally, the control of chemical processes is

accomplished by well-established procedures, e.g. by

changing the temperature or pressure of the reaction

mixture, or perhaps by using a catalyst that signifi-

cantly lower the activation energy for a given reaction

channel.

However, since the advent of laser technology, the

laser has been suggested as a new tool for controlling

chemical reactions. One of the most developed

schemes to control chemical reactions is through the

excitation of the reagents into specific states, which

are then stimulated to evolve into distinct product

states. An example of this line of attack has been

the development of mode-selective chemistry: for

certain reactions, vibrational excitation seems to be

more effective than translational excitation of the

reagents. However, it has to be noted that the rapid

internal vibrational redistribution within bond-

excited reagents makes mode selectivity in chemical

reactions a challenging task.

For the last decade or so, a new method has been

developed tocontrol chemical reactions that it isbased

on the wave nature of atoms and molecules. The

new methodology is called ‘quantum control’, or

‘coherent control’ of chemical reactions, and is

based on the coherent excitation of the molecule by

a laser. Generally speaking, an ultra-short laser pulse

creates a wave packet whose time evolution describes

the molecular evolution in the superposition of

excited states. Quantum control tries to modify the

superposition of such an ensemble of excited states

and, therefore, influences the motion of the wave

packet in such a manner that highly constructive

interference occurs in the desired reaction pathway,

and all other reaction pathways experience maximum

destructive interference.

The multidisciplinarity of laser chemistry

The rapid developments in new laser techniques

and applications have extended the field of laser

chemistry into many other scientific fields, such as

biology, medicine, and environmental science, as

well as into modern technological processes. This

‘natural’ invasion is a result of the multidisciplinary

character of modern laser chemistry. Examples of

this multidisciplinary character are numerous and

are amply covered in later chapters of the book.

However, a few examples are outlined here in order

to illustrate the key features relevant to laser chem-

istry better.

We have mentioned earlier that the brightness of

laser light provides the ideal conditions for non-linear

spectroscopy in atomic and molecular physics and

analytical chemistry, but it can also lead to ‘blood-

free’ and sterile surgery in medicine and other appli-

cation in modern biomedicine.

In surgery it is very important to achieve three main

effects: vaporization, coagulation and incision. The

experience gained through laser chemistry, particu-

larly with laser ablation of solid samples, has enabled

the laser beam parameters to be optimized for all three

effects. The photoactivation of certain chemicals in

vivo can be used in the treatment of cancer. As

described in Part 6 of this book, by photoactivating a

dye material (given to the patient sometime earlier to

the anticipated laser exposure) a photochemical reac-

tion can be initiated that causes the death of malignant

cells without destroying adjacent normal cells. This

treatment, known as photodynamic therapy, is a clear

example of the way in which laser-induced selective

chemistry can be used in medicine.

Laser analytical chemistry is perhaps one of the

sub-fields that has had the highest impact on other

fields and associated technologies. The spectral pur-

ity, or monochromaticity, of the laser light, amply

exploited in reaction dynamics by preparing specific

reagents’ states or probing specific product states, is

today used extensively in environmental science, e.g.

1.1 BASIC CONCEPTS IN LASER CHEMISTRY 9

for laser remote sensing (light detection and ranging

(lidar), differential absorption lidar (DIAL), etc.), or

in biology for selective excitation of chromophores in

cells or biological tissues. Key examples of these

types of multidisciplinary application are amply

described in later chapters.

Another illustration of the multidisciplinarity of

laser chemistry is the development of modern appli-

cations in nanotechnology, where, for example,

nanoscale patterning is an emerging laser chemical

method. We will see how the concept of localized

atomic scattering extends to that of localized atomic

reaction: the formation of the new bond created

at the surface takes place in an adjacent location

to the old bond that is being broken. In Chapter 27

we will then see how this localized atomic reaction

development can be used to produce nanoscale

patterning, i.e. patterning with exceptionally high

spatial resolution.

As mentioned above, the temporal coherence of the

laser light has revolutionized the investigation of che-

mical processes in real time because it has made

possible the preparation, and subsequent evolution,

of wave packets in molecular and atomic systems.

This coherent character of laser light is currently

used for quantum control of chemical processes.

Although this field is still in its infancy, important

scientific and technological applications are expected

in the near future and will undoubtedly extend beyond

chemistry.

One of the main applications of laser light in chem-

istry is the induction of chemical processes via stimu-

lated resonant transitions. The rate of excitation for a

stimulated transition is proportional to the light inten-

sity. Therefore, the use of intense laser light can

provide a very high rate of energy deposition into a

molecular system. Typical values can be 1–10 eV

during time periods of �10 ns down to less than

100 fs, i.e. up to 1014 eV s�1, which exceeds signifi-

cantly the system relaxation rate. This means that one

can excite atomic or molecular systems without any

‘heating’. These are ideal conditions with which to

develop mode-selective or bond-selective chemistry.

This has been a long-standing dream in chemistry,

whose realization has now become possible, albeit

only for certain restricted experimental conditions.

On the more practical side, high rates of light energy

deposition are now exploited in modern microbiology

or in the food industry (e.g. to sterilize solutions and

food products).

1.2 Organization of the book

The basic questions to be answered in any chemistry

experiment, or indeed any theoretical investigation,

are why and how chemical reactions (unimolecular or

bimolecular) occur. With laser chemistry one hopes to

elucidatewhether the presence of laser radiation in the

reaction zone influences the reaction by its interaction

with the reagents or reaction intermediates, or

whether it only serves as a probe to establish the

presence of a particular species in the entrance, inter-

mediate or exit channels of the reaction. These funda-

mental objectives, which are germane to the

understanding of laser chemistry, are detailed in this

textbook, together with a wealth of representative

examples.

Introduction to lasers, laser spectroscopy,instrumentation and measurementmethodology

Conceptually, all laser chemistry experiments are

made up of the same general building blocks, as

summarized in Figure 1.8.

At the centre of any laser chemistry experiment is

the reaction zone, on which normally all interest and

instrumental efforts are focused. Specific configura-

tions of the reaction region, and the experimental

apparatus used, differ widely; these depend on the

nature of the chemical reactants, how they are pre-

pared for interaction and what answers are sought in a

particular investigation. Hence, in this chapter, the

discussion of specific components (like vacuum

chambers, flow systems, particle beam generation,

etc.) are largely omitted (further details are given

where specific examples are discussed in later chap-

ters).

Around the reaction zone one can identify input and

output channels for atoms/molecules and radiation.

Broadly speaking, the input channel(s) for atoms and

molecules constitute the provision of reagents to the

reaction zone. This provision may happen in a variety

10 CH1 INTRODUCTION

of ways, including gas flows, atomic/molecular beam

transport or laser ablation, to name the most common

procedures encountered in experiments related to the

chemical reactions discussed in this book, i.e. reac-

tions mainly in the gas phase. Details of the relevant

mechanisms for atom/molecule provision and char-

acteristics of their motion are outlined later in the

chapters on unimolecular and bimolecular reactions

in Parts 4 and 5.

One can distinguish two main input channels for

laser radiation, namely one for the preparation of

reagents or reaction intermediates and one for the

probing of individual parts or the whole of the reac-

tion, from reagents through intermediates to products.

Both channels do not necessarily have to be present.

Depending on the nature of the experiment, one chan-

nel may be sufficient to provide the required informa-

tion, e.g. in cases in which a reaction is initiated by

laser radiation and its products are probed by non-

lasermeans,or, conversely,whereonly theproductsof

a chemical process are probed by a laser.

The results of a (laser-induced) chemical process

or the probing of a reaction are typically monitored

via the detection of photons or particles, or both. The

specific signatures of reagent/intermediate/product

responses are analysed using suitable ‘filters’ (e.g.

spectrometers for wavelength analysis of light, or

mass spectrometers for mass and/or energy analysis).

The response from the photon and particle detectors

following the analyser is ideally linear with incident

quanta, to allow for quantitative measurements.

Finally, in today’s high-tech world, the signals are

processed, accumulated and evaluated using a range

of computer-controlled equipment.

As is evident from Figure 1.8, many of the building

blocks in a general laser chemistry experiment

encompass several optical principles and comprise

numerous optical components. These include the

transfer and manipulation (intensity, spectral and tem-

poral characteristics) of light beams, significant parts

of the laser sources and spectral analysis equipment.

Hence, in order to avoid the reader having to revert

frequently to other textbooks, we provide in the first

three main sections (Parts 1–3):

1. A brief outline of basic information on the energy

levels in atoms and molecules, as well as photon

transitions/selection-rules (Chapter 2); a short

Figure 1.8 Conceptual set-up of a laser chemistry experiment, including channels for atomic and/or molecular injection,channels for laser-induced reagent preparation and laser probing, and channels for photon and atom/molecule detection andanalysis

1.2 ORGANIZATION OF THE BOOK 11

summary of how lasers work (Chapter 3) and

descriptions of the most common laser sources

used in laser chemistry studies (Chapter 4).

2. A detailed outline of those laser-spectroscopic

techniques, which are the most common in laser

chemistry experiments, including methods based

on photon detection (Chapters 6 to 8) and ion

detection (Chapter 9).

3. An introduction to the basic concepts of optical

phenomena and a description of routinely encoun-

tered optical components (Chapters 10 to 12). Part

3 concludes with a discussion of common photon

and atomic/molecular analysis systems, and the

associated detectors (Chapter 13), and a short

summary of signal processing and data acquisi-

tion, as far as it is relevant to the context of this

textbook (Chapter 14).

We have made every possible effort to keep these parts

as self-contained as possible; however, reference to

additional reading material isgivenwhere appropriate

or required.

Laser chemistry: unimolecular reactions,bimolecular reactions, cluster and surfacereactions

Central to this book is a second three-part set of

chapters where a wide range of laser chemistry prin-

ciples, processes and methodologies are discussed.

Numerous examples are provided, which highlight

specific aspects of particular principles or measure-

ment techniques. The following themes are discussed.

1. The concepts of laser chemistry are developed

along the lines of unimolecular reactions, or, in

other words, dissociative processes in the most

common sense. The discussion evolves from the

photodissociation of diatomic molecules through

triatomic species up to larger polyatomic entities

(Chapters 15 to 17). Suitable coverage is also given

to multiphoton and photoionization processes,

which involve the subtle inclusion of intermediate

and continuum states (Chapter 18). The part on

unimolecular reactions concludes with a discus-

sion of coherent control in chemical processes

(Chapter 19).

2. In the segment on bimolecular reactions, the

concepts of kinetics and reaction dynamics are

developed further; in particular, the ideas of

three-dimensional (3D) collision dynamics

and technologies (e.g. molecular beam techni-

ques) and the idea of state-to-state reactivity are

outlined (Chapters 20 and 21). The preparation

of reagents and the probing of reaction products

by laser techniques are extensively discussed in

Chapters 22 and 23.

3. Although the main focus is on gas-phase reactions,

the discussion would be incomplete without in-

cluding processes that are at the interface between

the different phases of matter (the boundaries of

chemical processes in the gas, liquid or solid phase

are indeed rather fuzzy). This area is addressed

in the segment on cluster and surface reactions,

evolving from van der Waals and cluster entities

(Chapter 24), via elementary reactions in a solvent

cage (Chapter 25), to laser-induced processes

in adsorbates on surfaces (Chapters 26 and 27).

In these three parts, emphasis is put on the under-

standing of fundamental principles; however, at the

same time, we have made every effort to cover modern

trends in the field of laser chemistry, e.g. the increas-

ing importance of femto-chemistry.

Practical applications

The fundamental processes and basic methodologies

in laser chemistry, which are the main focus of our

discussion, are now emerging from the realm of

curiosity-driven investigations into firmly based

applications in research laboratories and use in the

real world. Because of the rapid advance of laser

technology and the maturity of various laser chemical

techniques, the range of practical applications is

growing exponentially. Hence, this application

part can only provide snapshots, with a few selected

examples.

In the context of practical applications, laser chem-

istry reveals its inter- and multi-disciplinarity. The use

12 CH1 INTRODUCTION

of lasers in applications both driving and monitoring

chemical processes is found in fields such as the

following.

� Environmental studies, particularly of the atmo-

sphere; the primary chemistry of gas-phase reac-

tions, which is a centrepiece of this book, is most

evident and readily accessible (see Chapter 28).

� Combustion processes of small (e.g. car engines)

and large (e.g. incinerators) scale are the focus

of many studies, and instruments based on laser-

analytical techniques are now incorporated into

process control (see Chapter 29).

� Chemical processes are encountered in the biome-

dical context, and here the laser has helped to untan-

gle many of the extremely complex reaction chains

and study the underlying dynamics in real time

(see Chapter 30).

Worked examples and further material

Clearly, a textbook without some worked examples

and problems that a reader can try to solve, in order to

test his or her understanding of the material, would be

incomplete. However, the reader will immediately

notice, when scanning through the chapters, that

there are only a few worked examples, and no ques-

tion-and-answer sections at the end of a chapter, as is

common in many textbooks. This is deliberate and is

not an omission. When designing questions and

working through numerical examples, we found that

a large number of sensible problems needed the input

of data arrays, which would not have been easy to

incorporate into a written text. Therefore, we have

opted for an approach, that provides examples on the

web pages associated with the book (the pages are on

the Wiley web site www.wileyeurope.com/college/

Telle).

For each chapter we have collated a range of

questions and provided data. These will help the

reader gain deeper insight into many of the processes

we discuss. In addition, we have provided further

material in this form of electronic access, specifically

on topics that are very much in a state of flux today

(e.g. attosecond and free-electron laser sources). This

approach permits us to add or delete particular items

as they develop or lose mainstream interest. Also, we

have provided some additional material in colour;

although not essential, colour often makes particular

aspects easier to visualize, e.g. graphs, figures and

images.

1.2 ORGANIZATION OF THE BOOK 13